Business Combination, Consideration Transferred, Bond Exchanged, Fair Value

No definition available.

Name:	ago_BusinessCombinationConsiderationTransferredBondExchangedFairValue
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Business Combination, Recognized Identifiable Assets Acquired And Liabilities, Bargain Purchase And Settlement Of Pre-existing Relationship, Before Tax

+ References

No definition available.

+ Details

Name:	ago_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesBargainPurchaseAndSettlementOfPreexistingRelationshipBeforeTax
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Business Combination, Separately Recognized Transactions, Bargain Purchase And Settlement Of Pre-existing Relationship, Before Tax, Recognized

+ References

No definition available.

+ Details

Name:	ago_BusinessCombinationSeparatelyRecognizedTransactionsBargainPurchaseAndSettlementOfPreexistingRelationshipBeforeTaxRecognized
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, gross of ceded contracts.

+ References

No definition available.

+ Details

Name:	ago_InsuredFinancialObligationsOutstandingPrincipalAmountGross
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, after netting out ceded contracts.

+ References

No definition available.

+ Details

Name:	ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow associated with the acquisition of business during the period (for example, cash that was held by the acquired business).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

+ Details

Name:	us-gaap_CashAcquiredFromAcquisition
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ago_MBIAUKInsuranceLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=us-gaap_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ago_MBIAUKInsuranceLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Acquisitions - Narrative (Details) - USD ($) $ in Millions	Jan. 10, 2017	Mar. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Net par amount outstanding		$ 257,089	$ 264,952
MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Bargain purchase gain	$ 56
Gain on settlement of pre-existing relationships	2
Subsidiaries [Member] \| MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Cash expected to be acquired in acquisition	23
Bond exchanged, fair value	334
MBIA UK Insurance Limited [Member]
Business Acquisition [Line Items]
Gross Par Outstanding	347
Net par amount outstanding	$ 12,000